Your account appears, for whatever reason, bulk-created. For example maybe you were creating it from a network that somebody had used for a lot of IG account creation, or you created it with Firefox on Linux and 99% of their Firefox+Linux registrations are from spammers since it's easy to automate and run on a cloud server, etc.
It's actually pretty friendly from them to notify you immediately, rather than wait until you have gotten attached to the account.
> Are there really no less intrusive ways than the above to prove ones ownership of account??
Your ownership of the account is obviously not really under question. It's a freshly created account, it can't possibly have been hijacked yet. But is it your first account or the thousandth? Email addresses can be minted for free. Phone numbers for pennies. Since phone-verified, US-IP Instagram accounts seem to be selling for about $7 in bulk, those pennies are not much of an obstacle.
But it's pretty hard for you to get selfies from a lot of people in an automated way. (Sure, you could go to a parking lot and pay people to do the selfie for you. But that's a much higher bar.)
And then if suspicion remains, it allows IG to ask for either a second selfie or a picture of a government ID, and verify that your identity has actually stayed stable.